Neurons within the cat visual cortex show a great variety of receptive field properties. This functional diversity emerges mainly from interactions between thalamic and corticocortical inputs whose precise circuitry is largely unknown. In this proposal, two different approaches are taken to study the circuitry underlying the responses of complex cells in layer II-III. First, cross correlations, evaluated while recording from layer II-III, layer IV and the thalamus, are used to examine the cortical and/or thalamic inputs to complex cells. Second, small reversible blockades are made in the lateral geniculate nucleus (LGN) to study the contribution of different thalamic pathways to the generation of complex receptive fields. According to a hierarchical model of connectivity, the receptive fields of complex cells originates from the convergent inputs of layer IV simple cells. A major goal of this proposal is to examine the circuitry predicted by this model and investigate the role of different types of inputs, cortical and thalamic, in the generation of complex receptive fields. The study of the connections made by cells in the visual cortex is important to understand how neural systems extract information from the outside world. Only by such detailed knowledge of the cortical circuitry can neural disorders, such as many forms of amblyopia, be understood.